This invention relates to processes for the production of anionic polymers of styrenic monomers.
Polystyrene has many uses in the production of plastic articles and materials. For instance, brominated polystyrene is known to be a useful flame retardant for use in thermoplastics, e.g., polybutylene terephthalate, polyethylene terephthalate and nylon. The characteristics of the brominated polystyrene typically are determined by the process by which it is made. Polystyrene produced by anionic polymerization has been less preferred in the past because of its high cost and scarce availability. At least to some extent, these problems with respect to anionic polystyrene are a function of the complexity of the previously known processes for producing such a polymer. The processes previously employed, particularly those involving a batch operation, in the anionic polymerization of styrenic monomers have suffered from difficulties caused by the large exotherm created upon the initiation of the reaction being conducted, and from the generation of a product with high molecular weights and, in the case of cationic or free radical styrenic polymers, unfavorable polydispersity.
Thus, a need exists for a facile process for the production of anionic polymers of styrenic monomers which results in a product having suitable molecular weight and polydispersity characteristics. In the case of anionic styrenic polymers for use in the preparation of brominated styrenic flame retardants, it would be highly advantageous if a way could be found to produce an anionic styrenic polymer which is essentially free or free of olefinic and indane end groups which are common to cationic or free radical styrenic polymers. The avoidance of such end groups has been found to markedly increase the thermal stability of the resultant brominated styrenic polymer.
This invention is deemed to satisfy the foregoing needs in unique and elegant way by providing, amongst other things, a batch process for producing anionic styrenic polymer. The process avoids the use of aromatic solvents, such as benzene or toluene, and thus results in a polymer product substantially free of trace levels of such compounds, thereby avoiding the undesirable byproducts such impurities can create in downstream production of, e.g., brominated styrenic flame retardants. The process allows for higher reaction temperatures, as compared to previously known batch processes, while still controlling the process exotherm. Relative to previously known batch processes, lower amounts of ether promoter can be used in processes of this invention. This is especially advantageous when the desired product is a low molecular weight polymer because of the resulting economic benefits and the avoidance of deleterious effects of excessive promoter impurities in downstream products made from the polymer. The process comprises:
A) charging a liquid saturated hydrocarbon diluent and an ether promoter into a reactor; and then
B) either
1) (i) charging a saturated hydrocarbon solution of organolithium initiator into the reactor, in an amount to provide in the range of about 1 to about 10 mol % of organolithium initiator based on the total amount of a styrenic monomer to be added followed by (ii) the controlled addition of the styrenic monomer such that the temperature of the resultant reaction mixture is maintained at or below about 55xc2x0 C.; or
2) concurrently feeding separate feeds of (i) a styrenic monomer and (ii) a saturated hydrocarbon solution of organolithium initiator into the reactor, the feeds being maintained at rates to provide for the addition of an amount of organolithium initiator in the range of about 1 to about 10 mol % based on the total amount of styrenic monomer to be added, the temperature of the resultant reaction mixture being maintained at or below about 55xc2x0 C. and feed (ii) being of a shorter duration than feed (i).
In a preferred embodiment of this invention, batch process for producing anionic styrenic polymer is provided. The process comprises charging cyclohexane and an ether promoter into a reactor, and then prefeeding about 1 percent of the total amount of styrene monomer to the reactor, and then concurrently feeding separate feeds of (i) the remaining styrene monomer and (ii) a saturated hydrocarbon solution of organolithium initiator into the reactor. When operating on a scale of about 3,000 to about 6,000 lbs. of styrenic monomer, it is desirable to maintain the concurrent feeds over a period of time in the range of about 2 to about 10 minutes and at rates to provide for the addition of an amount of organolithium initiator in the range of about 2.5 to about 3.5 mol % based on the total amount of the styrene monomer. The temperature of the resultant reaction mixture is maintained at or below about 55xc2x0 C., and the styrene monomer is fed for a period of time not to exceed about 2 hours measured from initiation of the feeds (i) and (ii). The process is thus carried out so as to form an anionic styrenic polymer having a polydispersity index of about 1.2 or less.
These and still other embodiments, features and advantages of the present invention will become apparent from the following detailed description, examples and appended claims.
The styrenic monomer of this invention may be any anionically polymerizable styrenic monomer. Suitable non-limiting examples include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, (xcex1-methylstyrene, ethyl-styrene, tert-butylstyrene, dimethylstyrene, and the like including mixtures of two or more of the foregoing. Preferably, the styrenic monomer consists essentially of styrene.
The liquid saturated hydrocarbon diluent of this invention may be any aliphatic or cycloaliphatic hydrocarbon, or a mixture of two or more of the same, which is liquid under reaction conditions. The saturated hydrocarbon preferably contains in the range of about 4 to about 12 carbon atoms in the molecule. The aliphatic hydrocarbon may be linear or branched. Non-limiting examples of suitable aliphatic hydrocarbons include pentane, isopentane, hexane, 2-methylpentane, octane, 2,2,4-trimethylpentane and the like. More preferably, the liquid saturated hydrocarbon is one or more liquid saturated cycloaliphatic hydrocarbons. Suitable non-limiting examples of such cycloaliphatic hydrocarbons are cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane, cycloheptane, 1,3-dimethylcyclohexane, 1,4-dimethylcyclohexane and the like, with cyclohexane being particularly preferred as the liquid saturated hydrocarbon diluent.
The ether promoter of this invention may be a saturated aliphatic or cycloaliphatic monoether, a saturated aliphatic or cycloaliphatic diether or an aromatic ether. Thus, non-limiting examples of suitable ether promoters include tetrahydrofuran, methyl tert-butyl ether, ethyl tert-butyl ether, 1,4 dioxane, dimethoxyethane, methoxybenzene, and the like. The ether promoter is preferably a saturated cyclic or acyclic monoether having in the range of 4 to about 8 carbon atoms in the molecule. More preferably, the monoether is tetrahydrofuran (sometimes also referred to herein as xe2x80x9cTHFxe2x80x9d), methyltetrahydrofuran or dimethyltetrahydrofuran, or a mixture of any two or more of these. Tetrahydrofuran is particularly preferred. In another particularly preferred embodiment of this invention, the monoether consists essentially of an alkyl tert-butyl ether. Suitable alkyl tert-butyl ethers include, e.g., linear and branched chain alkyl tert-butyl ethers such as, e.g., methyl tert-butyl ether (sometimes also referred to herein as xe2x80x9cMTBExe2x80x9d) and ethyl tert-butyl ether, with methyl tert-butyl ether being particularly preferred. It is desirable to use an ether that is a liquid under the reaction conditions being used.
The organolithium initiator may be one of many lithium-containing hydrocarbons. Suitable non-limiting examples include methyllithium, ethyllithium, n- or sec-butyllithium, isopropyllithium, cyclohexyllithium orphenyllithium, including mixtures of the foregoing. n-Butyllithium and sec-butyllithium are preferred, with n-butyllithium being particularly preferred. The organolithium initiator is used in solution with a saturated hydrocarbon which may be the same or different from the liquid saturated hydrocarbon diluent, but is preferably the same.
The number average molecular weight of the polymer product made in accordance with this invention can vary, but will preferably be in the range of Mn 1000 to about Mn 10,000. The polymer product produced in accordance with this invention typically will have a polydispersity which is about 1.5 or less, and preferably is about 1.2 or less.
Prefeeding a portion of the styrenic monomer is not required for all embodiments of this invention, but is preferred because it appears to reduce the likelihood of reaction between the ether promoter and the organolithium initiator. The portion of the styrenic monomer which is prefer can vary, but typically will be about 1 percent by weight of the total amount of the styrenic monomer to be used in carrying out the reaction.
The feeds are maintained to provide preferably in the range of about 1 to about 10 mol %, more preferably about 2 to about 5 mol %, and most preferably in the range of about 2.5 to about 3.5 mol % of organolithium initiator based on the total amount of the styrenic monomer.
When bringing the reactants together in processes of this invention, one should use the minimum feed times while at the same time maintaining the temperature of the reaction mixture no higher than about 55xc2x0 C. For example, at a scale of about 4,000 lbs. of styrene feed, the feed rate of the organolithium feed is preferably about 2 to about 10 minutes, more preferably about 5 minutes, and the styrene co-feed should be effected in no more than about 2 hours, and more preferably within about 90 minutes or less, measured from initiation of co-feeding. However, when the monoether is methyl tert-butyl ether, at the foregoing scale, the styrene monomer co-feed preferably continues for a period of time not exceeding about 5 hours measured from initiation of the concurrent feeds. It will be noted that throughout this specification, including the appended claims, time periods provided are not scale dependent over the ranges of concentrations taught herein.
The reactor used in the process of this invention is typically equipped with a overhead heat exchanger. The process may be conducted at sub-atmospheric, atmospheric or super-atmospheric pressure. However, it is preferred to carry out the reaction at a reduced pressure, e.g., in the range from about 0.1 to about 0.7 atmospheres, so that the solvent is refluxed thereby providing consequent evaporative cooling of the highly exothermic reaction. The process of this invention is preferably conducted in the absence oxygen. Thus, the process should be carried out under an inert atmosphere such as, e.g., nitrogen or argon. The reaction system should be essentially anhydrous. By this is meant that small amounts of water insufficient to destroy the organolithium catalyst can be tolerated, but from a practical standpoint, the reaction equipment and reaction mixture should be kept as dry as reasonably practicable.
The temperature of the resultant reaction mixture is maintained at or below the specified temperature of about 55xc2x0 C. by any known method. For example, the reactor in which the reaction is conducted can be equipped with an external, indirect heat exchanger with a pump-around loop. The heat exchanger itself can be provided with a suitable coolant, e.g., a glycol coolant. Preferably, the reaction mixture is maintained at a temperature in the range of about 25xc2x0 C. to about 50xc2x0 C. After the feeds are terminated, the reaction mixture typically is held at the reaction temperature for about 5-10 minutes, e.g., when employing a scale of 3000-6000 lbs. of styrenic monomer, and then contacting the reaction mixture with an amount of water which is in the range of about 1.25 to about 10 moles of water, and preferably about 1.25 to about 5 moles of water per mole of organolithium originally charged, to quench the reaction and terminate the catalytic activity. By use of the process of this invention and termination using water, substantially all of the resultant polymer is characterized by having one of its end groups terminated by a proton (i.e., a hydrogen atom). Resulting lithium hydroxide salt hydrate is separated from the polymer solution by washing the reaction mixture with water, preferably in a 7:1 organic: aqueous weight ratio (accounting for water previously added). The aqueous phase which results is separated and the polymer-containing organic phase is devolatilized of the ether promoter and saturated hydrocarbon. Devolatization can be carried out in a variety of ways, including for example by pre-heating the mixture in a heat exchanger and feeding it into a hot (200xc2x0 C.) column under conditions such that the residual solvent and promoter are less than 0.5% by weight of the isolated polymer existing at the bottom of the column. The remaining polymer may then be dissolved in a suitable solvent, e.g., bromochloromethane, for storage.
Another way of terminating the reaction is to employ a lower alkyl halide, typically an alkyl chloride or an alkyl bromide, having in the range of 1 to about 8 carbon atoms. Use of an alkyl halide results in the formation of a styrenic polymer substantially all of which has one of its end groups terminated by an alkyl group rather than a proton. When using an alkyl halide to terminate the reaction, a stoichiometric amount, relative to the organolithium, should be employed. A feature of this embodiment is that the reaction product can remain substantially anhydrous since no water is added during production.
The amount of saturated hydrocarbon diluent and ether promoter employed in this invention may vary, but preferably is sufficient in the aggregate to cause the resultant reaction mixture to contain about 5 to about 70 wt %, and more preferably about 40 to about 60 wt %, of styrenic polymer upon termination of the styrene feed.
In another particularly preferred embodiment of this invention, styrenic polymer produced in accordance with the anionic polymerization process described above is placed in admixture with a brominating agent, such admixture being substantially free of a bromination catalyst, and fed to a catalytic quantity of a brominating agent. For further detailed teaching of such styrenic polymer bromination process, reference is made, for example, to U.S. Pat. No. 5,677,390, which is incorporated herein by reference.
In another particularly preferred embodiment of this invention, polystyrene produced in accordance with the anionic polymerization process described above is placed in solution and in admixture with a brominating agent, such admixture being substantially free of a bromination catalyst, and fed to a reactor containing a bromination catalyst and associated liquid, wherein the solvent used in forming the polystyrene solution and the liquid associated with the catalyst contains less than 200 ppm water between the two of them and the brominating agent contains less than about 100 ppm water. For detailed teaching of such polystyrene bromination process, reference is made, for example, to U.S. Pat. No. 5,852,132, which is incorporated herein by reference.
In another particularly preferred embodiment of this invention, styrenic polymer produced in accordance with the anionic polymerization process described above is contacted with a brominating agent in the presence of Lewis acid catalyst and solvent quantities of bromochloromethane. For further detailed teaching of such styrenic polymer bromination process, reference is made, for example, to U.S. Pat. No.5,767,203, which is incorporated herein by reference.
In another particularly preferred embodiment of this invention, polystyrene produced in accordance with the anionic polymerization process described above, bromochloromethane solvent and a Lewis acid catalyst are placed in a reaction vessel, and then a brominating agent is added to the vessel. Alternatively, the polystyrene is not placed in the reaction vessel initially; it is instead feed in admixture with the brominating agent to the reaction vessel which was previously charged with bromochloromethane solvent and a Lewis acid catalyst. For further detailed teaching of such polystyrene bromination process, reference is made, for example, to U.S. Pat. No. 5,916,978, which is incorporated herein by reference.
In another particularly preferred embodiment of this invention, polystyrene produced in accordance with the anionic polymerization process described above, bromochloromethane solvent and a Lewis acid catalyst are placed in a reaction vessel, and then a brominating agent is added to the vessel. Alternatively, the polystyrene is not placed in the reaction vessel initially; it is instead feed in admixture with the brominating agent to the reaction vessel which was previously charged with bromochloromethane solvent and a Lewis acid catalyst. For further detailed teaching of such polystyrene bromination process, reference is made, for example, to U.S. Pat. No. 5,916,978, which is incorporated herein by reference.
In another particularly preferred embodiment of this invention, a first stream comprising brominating agent, a second stream comprising anionic styrenic polymer formed as taught herein, and a third stream comprising bromination catalyst, are fed to a mixer to intimately mix such streams. For further detailed teaching of such styrenic polymer bromination process, reference is made, for example, to U.S. Pat. No. 5,686,538, which is incorporated herein by reference.
In another particularly preferred embodiment of this invention, a bromination catalyst and associated liquid are provided in a reactor and a portion of the bromination catalyst and associated liquid is fed to a mixer external of the reactor; a brominating agent and a solution of anionic polystyrene formed in accordance with the process taught herein are fed as separate streams to the external mixer, in which the separate streams, prior to being fed to the external mixer, are substantially free of a bromination catalyst, the solvent used in forming the polystyrene solution and the liquid associated with the catalyst contains less than 200 ppm water between the two of them, and the brominating agent contains less than about 100 ppm water. For further detailed teaching of such polystyrene bromination process, reference is made, for example, to U.S. Pat. No. 5,852,131, which is incorporated herein by reference.
In another particularly preferred embodiment of this invention, anionic polystyrene produced as described herein is mixed with a brominating agent and fed to a reaction vessel to which was previously added bromochloromethane solvent and a Lewis acid catalyst, the mole ratio of brominating agent to polystyrene in the feed mixture being from about 1:1 to about 8:1. For further detailed teaching of such polystyrene bromination process, reference is made, for example, to U.S. Pat. No. 6,207,765 B1, which is incorporated herein by reference.